Thanks to Hyeong-jun (H. J.) for emailing me this problem! This is a challenging problem from the math section of the 1997 CSAT, a standardized test in South Korea. Can you figure it out? It took me several attempts, but it was really satisfying when I solved it (I did need to look up one key insight, which is the first thing presented in the solution). So give it a try, and don't peek at the rest of the solution too early!

0:00 Problem
1:58 My many attempts
3:28 Unwrapping a cone
5:14 Solution

Coordinate geometry solution and proof of uphill/downhill track
https://www.desmos.com/calculator/l188uqapxo

Note: pretty much every time I said or wrote "circular arc" I meant to say or write "circular sector." A "circular arc" is a portion of a circle's circumference; a "circular sector" is the enclosed region between the arc and two radii.

Can You Solve This 6th Grade Geometry Problem From China? (1.7 million views)
https://www.youtube.com/watch?v=xnE_sO7PbBs

HARD Geometry Problem: Can You Solve The Horse Grazing Puzzle? (165,000 views)
https://www.youtube.com/watch?v=kaLiagYuYPc

Can You Solve A REALLY HARD Math Problem? The Circle Inscribed In A Parabola Puzzle (83,000 views)
https://www.youtube.com/watch?v=z2P8q4QC53s

My blog post for this video
https://wp.me/p6aMk-7UJ

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